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Retinanet网络与focal loss损失

news 2026/1/12 22:24:50

参考代码：https://github.com/yhenon/pytorch-retinanet

1.损失函数

1）原理

本文一个核心的贡献点就是 focal loss。总损失依然分为两部分，一部分是分类损失，一部分是回归损失。

在讲分类损失之前，我们来回顾一下二分类交叉熵损失（binary_cross_entropy）。

计算代码如下：

import numpy as np

y_true = np.array([0., 1., 1., 1., 1., 1., 1., 1., 1., 1.])
y_pred = np.array([0.2, 0.8, 0.8, 0.8, 0.8, 0.8, 0.8, 0.8, 0.8, 0.8])

my_loss = - y_true * np.log(y_pred) - (1 - y_true) * np.log(1 - y_pred)
mean_my_loss = np.mean(my_loss)
print("mean_my_loss:",mean_my_loss)

调用pytorch自带的函数计算

import torch.nn.functional as F
import numpy as np
import torch

torch_pred = torch.tensor(y_pred)
torch_true = torch.tensor(y_true)
bce_loss = F.binary_cross_entropy(torch_pred, torch_true)
print('bce_loss:', bce_loss)

现在回到focal loss，Focal loss的起源是二分类交叉熵。

二分类的交叉熵损失还可以如下表示，其中y∈{1，-1}，1代表候选框是正样本，-1代表是负样本：

为了表示方便，可以定义如下公式：

那么问题来了，应用场景如下：

在one-stage 物体检测模型中，一张图中能匹配到目标的候选框（正样本）大概是十几个到几十个，然后没有匹配到的候选框（负样本）10 的四次方到五次方。这些负样本中，大部分都是简单易分的样本，对于训练样本起不到作用，反而淹没了有助于训练的样本。

举个例子，正样本有50个，损失是3，负样本是10000个，损失是0.1

那么50x3 = 150，10000x0.1=1000

所以，为了平衡交叉熵，采用了系数αt，当是正样本的时候，αt = α，负样本的时候 αt=1-α，α∈[0,1]

αt能平衡正负样本的权重，但是不能区分哪些是困难样本，哪些是容易样本（是否对训练有帮助）。

所以继续引入公式，这样就解决了区分样本容易性的问题：

最后，结合两个公式，形成最终的公式。

展开形式如下

现在来看一下效果，p代表预测候选框是正样本的概率，y是候选框实际上是正样本还是负样本，CE是普通交叉熵计算的损失，FL是focal loss，rate是缩小的比例。可以看出，最后两行难区分样本的rate很小。

2）代码

import numpy as np
import torch
import torch.nn as nnclass FocalLoss(nn.Module):#def __init__(self):def forward(self, classifications, regressions, anchors, annotations):alpha = 0.25gamma = 2.0# classifications是预测结果batch_size = classifications.shape[0]# 分类lossclassification_losses = []# 回归lossregression_losses = []# anchors的形状是 [1, 每层anchor数量之和 , 4]anchor = anchors[0, :, :]anchor_widths  = anchor[:, 2] - anchor[:, 0]  # x2-x1anchor_heights = anchor[:, 3] - anchor[:, 1]  # y2-y1anchor_ctr_x   = anchor[:, 0] + 0.5 * anchor_widths  # 中心点x坐标anchor_ctr_y   = anchor[:, 1] + 0.5 * anchor_heights  # 中心点y坐标for j in range(batch_size):# classifications的shape [batch,所有anchor的数量，分类数]classification = classifications[j, :, :]# classifications的shape [batch,所有anchor的数量，分类数]regression = regressions[j, :, :]bbox_annotation = annotations[j, :, :]bbox_annotation = bbox_annotation[bbox_annotation[:, 4] != -1]classification = torch.clamp(classification, 1e-4, 1.0 - 1e-4)if bbox_annotation.shape[0] == 0:if torch.cuda.is_available():alpha_factor = torch.ones(classification.shape).cuda() * alphaalpha_factor = 1. - alpha_factorfocal_weight = classificationfocal_weight = alpha_factor * torch.pow(focal_weight, gamma)bce = -(torch.log(1.0 - classification))# cls_loss = focal_weight * torch.pow(bce, gamma)cls_loss = focal_weight * bceclassification_losses.append(cls_loss.sum())regression_losses.append(torch.tensor(0).float().cuda())else:alpha_factor = torch.ones(classification.shape) * alphaalpha_factor = 1. - alpha_factorfocal_weight = classificationfocal_weight = alpha_factor * torch.pow(focal_weight, gamma)bce = -(torch.log(1.0 - classification))# cls_loss = focal_weight * torch.pow(bce, gamma)cls_loss = focal_weight * bceclassification_losses.append(cls_loss.sum())regression_losses.append(torch.tensor(0).float())continue# 每个anchor 与 每个标注的真实框的iouIoU = calc_iou(anchors[0, :, :], bbox_annotation[:, :4]) # num_anchors x num_annotations# 每个anchor对应的最大的iou （anchor与grandtruce进行配对）# 得到了配对的索引和对应的最大值IoU_max, IoU_argmax = torch.max(IoU, dim=1)#import pdb#pdb.set_trace()# compute the loss for classification# classification 的shape[anchor总数，分类数]targets = torch.ones(classification.shape) * -1if torch.cuda.is_available():targets = targets.cuda()# 判断每个元素是否小于0.4 小于就返回true（anchor对应的最大iou<0.4,那就是背景）targets[torch.lt(IoU_max, 0.4), :] = 0# 最大iou大于0.5的anchor索引positive_indices = torch.ge(IoU_max, 0.5)num_positive_anchors = positive_indices.sum()assigned_annotations = bbox_annotation[IoU_argmax, :]targets[positive_indices, :] = 0targets[positive_indices, assigned_annotations[positive_indices, 4].long()] = 1if torch.cuda.is_available():alpha_factor = torch.ones(targets.shape).cuda() * alphaelse:alpha_factor = torch.ones(targets.shape) * alphaalpha_factor = torch.where(torch.eq(targets, 1.), alpha_factor, 1. - alpha_factor)focal_weight = torch.where(torch.eq(targets, 1.), 1. - classification, classification)focal_weight = alpha_factor * torch.pow(focal_weight, gamma)bce = -(targets * torch.log(classification) + (1.0 - targets) * torch.log(1.0 - classification))# cls_loss = focal_weight * torch.pow(bce, gamma)cls_loss = focal_weight * bceif torch.cuda.is_available():cls_loss = torch.where(torch.ne(targets, -1.0), cls_loss, torch.zeros(cls_loss.shape).cuda())else:cls_loss = torch.where(torch.ne(targets, -1.0), cls_loss, torch.zeros(cls_loss.shape))classification_losses.append(cls_loss.sum()/torch.clamp(num_positive_anchors.float(), min=1.0))# compute the loss for regressionif positive_indices.sum() > 0:assigned_annotations = assigned_annotations[positive_indices, :]anchor_widths_pi = anchor_widths[positive_indices]anchor_heights_pi = anchor_heights[positive_indices]anchor_ctr_x_pi = anchor_ctr_x[positive_indices]anchor_ctr_y_pi = anchor_ctr_y[positive_indices]gt_widths  = assigned_annotations[:, 2] - assigned_annotations[:, 0]gt_heights = assigned_annotations[:, 3] - assigned_annotations[:, 1]gt_ctr_x   = assigned_annotations[:, 0] + 0.5 * gt_widthsgt_ctr_y   = assigned_annotations[:, 1] + 0.5 * gt_heights# clip widths to 1gt_widths  = torch.clamp(gt_widths, min=1)gt_heights = torch.clamp(gt_heights, min=1)targets_dx = (gt_ctr_x - anchor_ctr_x_pi) / anchor_widths_pitargets_dy = (gt_ctr_y - anchor_ctr_y_pi) / anchor_heights_pitargets_dw = torch.log(gt_widths / anchor_widths_pi)targets_dh = torch.log(gt_heights / anchor_heights_pi)targets = torch.stack((targets_dx, targets_dy, targets_dw, targets_dh))targets = targets.t()if torch.cuda.is_available():targets = targets/torch.Tensor([[0.1, 0.1, 0.2, 0.2]]).cuda()else:targets = targets/torch.Tensor([[0.1, 0.1, 0.2, 0.2]])negative_indices = 1 + (~positive_indices)regression_diff = torch.abs(targets - regression[positive_indices, :])regression_loss = torch.where(torch.le(regression_diff, 1.0 / 9.0),0.5 * 9.0 * torch.pow(regression_diff, 2),regression_diff - 0.5 / 9.0)regression_losses.append(regression_loss.mean())else:if torch.cuda.is_available():regression_losses.append(torch.tensor(0).float().cuda())else:regression_losses.append(torch.tensor(0).float())return torch.stack(classification_losses).mean(dim=0, keepdim=True), torch.stack(regression_losses).mean(dim=0, keepdim=True)

3）分类损失的计算过程

假设一张图片有n个anchor，有m个grandtrue，有L个类别

1.得到anchor和每一个grandtrue的IOU

# 每个anchor 与 每个标注的真实框的iou
IoU = calc_iou(anchors[0, :, :], bbox_annotation[:, :4]) # num_anchors x num_annotations

2.得到每个anchor最大的IOU，以及对应的grandtrue

IoU_max, IoU_argmax = torch.max(IoU, dim=1)

3.初始化一个分类目标结果表，默认值是-1

 targets = torch.ones(classification.shape) * -1

4.如果某个anchor的最大IOU<0.4，那么它对应的分类全为0

targets[torch.lt(IoU_max, 0.4), :] = 0

例如：iou3m = 0.3，ioun2 = 0.2

此时，上述分类结果表就更新anchor3，和anchorn的分类结果

5.把每个anchor关联对应的grandtruce信息，其中参数5是预测的类别

# 最大iou大于0.5的anchor索引
positive_indices = torch.ge(IoU_max, 0.5)num_positive_anchors = positive_indices.sum()assigned_annotations = bbox_annotation[IoU_argmax, :]

6.如果anchor的最大IOU>0.5,那么根据参数5，修改对应的分类结果表为one-hot形式

targets[positive_indices, :] = 0
targets[positive_indices, assigned_annotations[positive_indices, 4].long()] = 1

例如 iou12 = 0.6，参数5 = class2

修改分类结果表

7. 得到损失的权重部分

alpha_factor = torch.where(torch.eq(targets, 1.), alpha_factor, 1. - alpha_factor)
focal_weight = torch.where(torch.eq(targets, 1.), 1. - classification, classification)
focal_weight = alpha_factor * torch.pow(focal_weight, gamma)

α表，将0的地方替换成1-α，1的地方替换成 α

p表将0的地方原概率，1的地方换成1-p

权重表的元素就是两表对应元素的乘积

8.得到损失的损失部分

bce = -(targets * torch.log(classification) + (1.0 - targets) * torch.log(1.0 - classification))

9.得到初步的损失结果

cls_loss = focal_weight * bce

10.将分类结果表原本是-1的地方，对应的损失变成0

例如anchor2最大iou是0.45，介于0.4与0.5之间，我们就不计算他的损失，忽略不计

if torch.cuda.is_available():cls_loss = torch.where(torch.ne(targets, -1.0), cls_loss, torch.zeros(cls_loss.shape).cuda())
else:cls_loss = torch.where(torch.ne(targets, -1.0), cls_loss, torch.zeros(cls_loss.shape))

11.损失汇总

classification_losses.append(cls_loss.sum()/torch.clamp(num_positive_anchors.float(), min=1.0))

2.网络结构

整体来讲，网络采用了FPN模型

这个结构也是可以变的（可以灵活改变），如下所示

模型如下所示

其中每个位置的anchor是9个，三个形状x三个比例

K是分类的数量，A是每个位置anchor是数量

4A，4是四个参数可以确定anchor的位置和大小。

3.代码讲解：

1.FPN分支部分

self.P5_1 = nn.Conv2d(C5_size, feature_size, kernel_size=1, stride=1, padding=0)
self.P5_upsampled = nn.Upsample(scale_factor=2, mode='nearest')
self.P5_2 = nn.Conv2d(feature_size, feature_size, kernel_size=3, stride=1, padding=1)


self.P4_1 = nn.Conv2d(C4_size, feature_size, kernel_size=1, stride=1, padding=0)
self.P4_upsampled = nn.Upsample(scale_factor=2, mode='nearest')
self.P4_2 = nn.Conv2d(feature_size, feature_size, kernel_size=3, stride=1, padding=1)

self.P3_1 = nn.Conv2d(C3_size, feature_size, kernel_size=1, stride=1, padding=0)
self.P3_2 = nn.Conv2d(feature_size, feature_size, kernel_size=3, stride=1, padding=1)

self.P6 = nn.Conv2d(C5_size, feature_size, kernel_size=3, stride=2, padding=1)

self.P7_1 = nn.ReLU()
self.P7_2 = nn.Conv2d(feature_size, feature_size, kernel_size=3, stride=2, padding=1)

forward

    def forward(self, inputs):#inputs 是主干模块conv3、conv4、conv5的输出C3, C4, C5 = inputsP5_x = self.P5_1(C5)P5_upsampled_x = self.P5_upsampled(P5_x)P5_x = self.P5_2(P5_x)P4_x = self.P4_1(C4)P4_x = P5_upsampled_x + P4_xP4_upsampled_x = self.P4_upsampled(P4_x)P4_x = self.P4_2(P4_x)P3_x = self.P3_1(C3)P3_x = P3_x + P4_upsampled_xP3_x = self.P3_2(P3_x)P6_x = self.P6(C5)P7_x = self.P7_1(P6_x)P7_x = self.P7_2(P7_x)return [P3_x, P4_x, P5_x, P6_x, P7_x]

2.回归自网络

class RegressionModel(nn.Module):def __init__(self, num_features_in, num_anchors=9, feature_size=256):super(RegressionModel, self).__init__()#其实num_features_in就等于256self.conv1 = nn.Conv2d(num_features_in, feature_size, kernel_size=3, padding=1)self.act1 = nn.ReLU()self.conv2 = nn.Conv2d(feature_size, feature_size, kernel_size=3, padding=1)self.act2 = nn.ReLU()self.conv3 = nn.Conv2d(feature_size, feature_size, kernel_size=3, padding=1)self.act3 = nn.ReLU()self.conv4 = nn.Conv2d(feature_size, feature_size, kernel_size=3, padding=1)self.act4 = nn.ReLU()#4个参数就能确定anchor的大小self.output = nn.Conv2d(feature_size, num_anchors * 4, kernel_size=3, padding=1)def forward(self, x):out = self.conv1(x)out = self.act1(out)out = self.conv2(out)out = self.act2(out)out = self.conv3(out)out = self.act3(out)out = self.conv4(out)out = self.act4(out)out = self.output(out)# out is B x C x W x H, with C = 4*num_anchorsout = out.permute(0, 2, 3, 1)#相当于展平了，-1的位置相当于所有anchor的数目return out.contiguous().view(out.shape[0], -1, 4)

3.分类网络

class ClassificationModel(nn.Module):def __init__(self, num_features_in, num_anchors=9, num_classes=80, prior=0.01, feature_size=256):super(ClassificationModel, self).__init__()self.num_classes = num_classesself.num_anchors = num_anchorsself.conv1 = nn.Conv2d(num_features_in, feature_size, kernel_size=3, padding=1)self.act1 = nn.ReLU()self.conv2 = nn.Conv2d(feature_size, feature_size, kernel_size=3, padding=1)self.act2 = nn.ReLU()self.conv3 = nn.Conv2d(feature_size, feature_size, kernel_size=3, padding=1)self.act3 = nn.ReLU()self.conv4 = nn.Conv2d(feature_size, feature_size, kernel_size=3, padding=1)self.act4 = nn.ReLU()self.output = nn.Conv2d(feature_size, num_anchors * num_classes, kernel_size=3, padding=1)self.output_act = nn.Sigmoid()def forward(self, x):out = self.conv1(x)out = self.act1(out)out = self.conv2(out)out = self.act2(out)out = self.conv3(out)out = self.act3(out)out = self.conv4(out)out = self.act4(out)out = self.output(out)out = self.output_act(out)# out is B x C x W x H, with C = n_classes + n_anchorsout1 = out.permute(0, 2, 3, 1)batch_size, width, height, channels = out1.shapeout2 = out1.view(batch_size, width, height, self.num_anchors, self.num_classes)return out2.contiguous().view(x.shape[0], -1, self.num_classes)

4.主干网络、训练和预测过程

1.网络结构

经过conv1缩小4倍，经过conv2不变，conv3、v4，v5都缩小两倍，p5到p6缩小两倍，p6到p7缩小两倍

p3相对于图片缩小了2的3次方，p4相对于图片缩小了2的4次方，以此类推

class ResNet(nn.Module):#layers是层数def __init__(self, num_classes, block, layers):self.inplanes = 64super(ResNet, self).__init__()#这个是输入 conv1self.conv1 = nn.Conv2d(3, 64, kernel_size=7, stride=2, padding=3, bias=False)self.bn1 = nn.BatchNorm2d(64)self.relu = nn.ReLU(inplace=True)self.maxpool = nn.MaxPool2d(kernel_size=3, stride=2, padding=1)#这是c2self.layer1 = self._make_layer(block, 64, layers[0])#这是c3self.layer2 = self._make_layer(block, 128, layers[1], stride=2)#这是c4self.layer3 = self._make_layer(block, 256, layers[2], stride=2)#这是c5self.layer4 = self._make_layer(block, 512, layers[3], stride=2)#得到c3、c4、c5输出的通道数if block == BasicBlock:fpn_sizes = [self.layer2[layers[1] - 1].conv2.out_channels, self.layer3[layers[2] - 1].conv2.out_channels,self.layer4[layers[3] - 1].conv2.out_channels]elif block == Bottleneck:fpn_sizes = [self.layer2[layers[1] - 1].conv3.out_channels, self.layer3[layers[2] - 1].conv3.out_channels,self.layer4[layers[3] - 1].conv3.out_channels]else:raise ValueError(f"Block type {block} not understood")#创建FPN的分支部分self.fpn = PyramidFeatures(fpn_sizes[0], fpn_sizes[1], fpn_sizes[2])#创建回归网络self.regressionModel = RegressionModel(256)#创建分类网络self.classificationModel = ClassificationModel(256, num_classes=num_classes)self.anchors = Anchors()self.regressBoxes = BBoxTransform()self.clipBoxes = ClipBoxes()self.focalLoss = losses.FocalLoss()#权重初始化for m in self.modules():if isinstance(m, nn.Conv2d):n = m.kernel_size[0] * m.kernel_size[1] * m.out_channelsm.weight.data.normal_(0, math.sqrt(2. / n))elif isinstance(m, nn.BatchNorm2d):m.weight.data.fill_(1)m.bias.data.zero_()prior = 0.01self.classificationModel.output.weight.data.fill_(0)self.classificationModel.output.bias.data.fill_(-math.log((1.0 - prior) / prior))self.regressionModel.output.weight.data.fill_(0)self.regressionModel.output.bias.data.fill_(0)#冻结bn层参数更新，因为预训练的参数已经很好了self.freeze_bn()def _make_layer(self, block, planes, blocks, stride=1):downsample = Noneif stride != 1 or self.inplanes != planes * block.expansion:downsample = nn.Sequential(nn.Conv2d(self.inplanes, planes * block.expansion,kernel_size=1, stride=stride, bias=False),nn.BatchNorm2d(planes * block.expansion),)layers = [block(self.inplanes, planes, stride, downsample)]self.inplanes = planes * block.expansionfor i in range(1, blocks):layers.append(block(self.inplanes, planes))return nn.Sequential(*layers)def freeze_bn(self):'''Freeze BatchNorm layers.'''for layer in self.modules():if isinstance(layer, nn.BatchNorm2d):layer.eval()

2.训练过程和预测过程

1）anchor的调整

生成的预测值 regression [batch, anchor的数量，4] regression[:, :, 0]和[:, :, 1]用来移动anchor中心点 [:, :, 2]和[:, :, 3]用来改变框子的长度

import torch.nn as nn
import torch
import numpy as np# 生成的预测值 regression [batch, anchor的数量，4]  regression[:, :, 0]和[:, :, 1]用来移动anchor中心点 [:, :, 2]和[:, :, 3]用来改变框子的长度
class BBoxTransform(nn.Module):def __init__(self, mean=None, std=None):super(BBoxTransform, self).__init__()if mean is None:if torch.cuda.is_available():self.mean = torch.from_numpy(np.array([0, 0, 0, 0]).astype(np.float32)).cuda()else:self.mean = torch.from_numpy(np.array([0, 0, 0, 0]).astype(np.float32))else:self.mean = meanif std is None:if torch.cuda.is_available():self.std = torch.from_numpy(np.array([0.1, 0.1, 0.2, 0.2]).astype(np.float32)).cuda()else:self.std = torch.from_numpy(np.array([0.1, 0.1, 0.2, 0.2]).astype(np.float32))else:self.std = stddef forward(self, boxes, deltas):#boxes就是图片所有的anchor[batch ， 一张图片上anchor的总数 ，4]widths  = boxes[:, :, 2] - boxes[:, :, 0]  # x2 - x1 = 宽heights = boxes[:, :, 3] - boxes[:, :, 1]  # y2 - y1 = 高ctr_x   = boxes[:, :, 0] + 0.5 * widths    # x1 + 宽/2 = 中心点 xctr_y   = boxes[:, :, 1] + 0.5 * heights   # y1 + 高/2 = 中心点 ydx = deltas[:, :, 0] * self.std[0] + self.mean[0]dy = deltas[:, :, 1] * self.std[1] + self.mean[1]dw = deltas[:, :, 2] * self.std[2] + self.mean[2]dh = deltas[:, :, 3] * self.std[3] + self.mean[3]pred_ctr_x = ctr_x + dx * widthspred_ctr_y = ctr_y + dy * heightspred_w     = torch.exp(dw) * widthspred_h     = torch.exp(dh) * heightspred_boxes_x1 = pred_ctr_x - 0.5 * pred_wpred_boxes_y1 = pred_ctr_y - 0.5 * pred_hpred_boxes_x2 = pred_ctr_x + 0.5 * pred_wpred_boxes_y2 = pred_ctr_y + 0.5 * pred_hpred_boxes = torch.stack([pred_boxes_x1, pred_boxes_y1, pred_boxes_x2, pred_boxes_y2], dim=2)return pred_boxes

2总过程

    def forward(self, inputs):if self.training:img_batch, annotations = inputselse:img_batch = inputsx = self.conv1(img_batch)x = self.bn1(x)x = self.relu(x)x = self.maxpool(x)x1 = self.layer1(x)x2 = self.layer2(x1)x3 = self.layer3(x2)x4 = self.layer4(x3)features = self.fpn([x2, x3, x4])#shape[batch,每次anchor总数之和，4个值]regression = torch.cat([self.regressionModel(feature) for feature in features], dim=1)# shape[batch,每次anchor总数之和，分类个数]classification = torch.cat([self.classificationModel(feature) for feature in features], dim=1)anchors = self.anchors(img_batch)if self.training:return self.focalLoss(classification, regression, anchors, annotations)else:#得到调节参数之后的框子transformed_anchors = self.regressBoxes(anchors, regression)#保证框子在图片之内transformed_anchors = self.clipBoxes(transformed_anchors, img_batch)finalResult = [[], [], []]#每个框对应类别的置信度finalScores = torch.Tensor([])#框对应的分类序号：第几类finalAnchorBoxesIndexes = torch.Tensor([]).long()#框的坐标finalAnchorBoxesCoordinates = torch.Tensor([])if torch.cuda.is_available():finalScores = finalScores.cuda()finalAnchorBoxesIndexes = finalAnchorBoxesIndexes.cuda()finalAnchorBoxesCoordinates = finalAnchorBoxesCoordinates.cuda()for i in range(classification.shape[2]):scores = torch.squeeze(classification[:, :, i])scores_over_thresh = (scores > 0.05)if scores_over_thresh.sum() == 0:# no boxes to NMS, just continuecontinuescores = scores[scores_over_thresh]anchorBoxes = torch.squeeze(transformed_anchors)anchorBoxes = anchorBoxes[scores_over_thresh]anchors_nms_idx = nms(anchorBoxes, scores, 0.5)finalResult[0].extend(scores[anchors_nms_idx])finalResult[1].extend(torch.tensor([i] * anchors_nms_idx.shape[0]))finalResult[2].extend(anchorBoxes[anchors_nms_idx])finalScores = torch.cat((finalScores, scores[anchors_nms_idx]))finalAnchorBoxesIndexesValue = torch.tensor([i] * anchors_nms_idx.shape[0])if torch.cuda.is_available():finalAnchorBoxesIndexesValue = finalAnchorBoxesIndexesValue.cuda()finalAnchorBoxesIndexes = torch.cat((finalAnchorBoxesIndexes, finalAnchorBoxesIndexesValue))finalAnchorBoxesCoordinates = torch.cat((finalAnchorBoxesCoordinates, anchorBoxes[anchors_nms_idx]))return [finalScores, finalAnchorBoxesIndexes, finalAnchorBoxesCoordinates]

5.两种block的定义

import torch.nn as nndef conv3x3(in_planes, out_planes, stride=1):"""3x3 convolution with padding"""return nn.Conv2d(in_planes, out_planes, kernel_size=3, stride=stride,padding=1, bias=False)class BasicBlock(nn.Module):expansion = 1def __init__(self, inplanes, planes, stride=1, downsample=None):super(BasicBlock, self).__init__()self.conv1 = conv3x3(inplanes, planes, stride)self.bn1 = nn.BatchNorm2d(planes)self.relu = nn.ReLU(inplace=True)self.conv2 = conv3x3(planes, planes)self.bn2 = nn.BatchNorm2d(planes)self.downsample = downsampleself.stride = stridedef forward(self, x):residual = xout = self.conv1(x)out = self.bn1(out)out = self.relu(out)out = self.conv2(out)out = self.bn2(out)if self.downsample is not None:residual = self.downsample(x)out += residualout = self.relu(out)return outclass Bottleneck(nn.Module):expansion = 4def __init__(self, inplanes, planes, stride=1, downsample=None):super(Bottleneck, self).__init__()self.conv1 = nn.Conv2d(inplanes, planes, kernel_size=1, bias=False)self.bn1 = nn.BatchNorm2d(planes)self.conv2 = nn.Conv2d(planes, planes, kernel_size=3, stride=stride,padding=1, bias=False)self.bn2 = nn.BatchNorm2d(planes)self.conv3 = nn.Conv2d(planes, planes * 4, kernel_size=1, bias=False)self.bn3 = nn.BatchNorm2d(planes * 4)self.relu = nn.ReLU(inplace=True)self.downsample = downsampleself.stride = stridedef forward(self, x):residual = xout = self.conv1(x)out = self.bn1(out)out = self.relu(out)out = self.conv2(out)out = self.bn2(out)out = self.relu(out)out = self.conv3(out)out = self.bn3(out)if self.downsample is not None:residual = self.downsample(x)out += residualout = self.relu(out)return out

6.anchor

1.每个位置的生成anchor函数

anchor的生成都是以原图为基准的
这个函数的作用是生成一个位置中所有的anchor，形式是（x1，y1，x2，y2）并且（X1，y1）和（x2，y2）关于中心对称，这样给定一个中点，可以直接拿（x1，y1，x2，y2）计算出相应的anchor

大概功能步骤：

1.确定每个位置anchor的数量：宽高比例数量x边长缩放比例数量

2.得到anchor的标准边长缩放后的结果：base_size x scales

3.通过上述结果得到标准面积：（base_size x scales）的平方

2.通过h = sqrt(areas / ratio)和w = h * ratio得到宽高

3.得到每个anchor的两个坐标 (0-h/2 , 0-w/2) 和 (h/2 , w/2)

4.输出anchor

def generate_anchors(base_size=16, ratios=None, scales=None):"""Generate anchor (reference) windows by enumerating aspect ratios Xscales w.r.t. a reference window."""if ratios is None:ratios = np.array([0.5, 1, 2])if scales is None:scales = np.array([2 ** 0, 2 ** (1.0 / 3.0), 2 ** (2.0 / 3.0)])#每个位置的anchor总数 n种规模 * m种比例num_anchors = len(ratios) * len(scales)# 初始化anchor的参数 x，y，w，hanchors = np.zeros((num_anchors, 4))# scale base_size#np.tile(scales, (2, len(ratios))).T结果如下：#[[1.         1.        ]# [1.25992105 1.25992105]# [1.58740105 1.58740105]# [1.         1.        ]# [1.25992105 1.25992105]# [1.58740105 1.58740105]# [1.         1.        ]# [1.25992105 1.25992105]# [1.58740105 1.58740105]]# shape (9, 2)#设置anchor的w、h的基础大小（1：1）anchors[:, 2:] = base_size * np.tile(scales, (2, len(ratios))).T# 计算anchor的基础面积#[area1,area2,area3,area1,area2,area3,area1,area2,area3]areas = anchors[:, 2] * anchors[:, 3]# correct for ratios#利用面积和宽高比得到真正的宽和高#根据公式1: areas / (w/h) = areas / ratio = hxh  => h = sqrt(areas / ratio)# 公式2：w = h * ratio#np.repeat(ratios, len(scales))) = [0.5,0.5,0.5 ,1,1,1,2,2,2]# 最终的效果就是 面积1的高宽，面积2的高宽，面积3的高宽anchors[:, 2] = np.sqrt(areas / np.repeat(ratios, len(scales)))anchors[:, 3] = anchors[:, 2] * np.repeat(ratios, len(scales))# 转换anchor的形式 (x_ctr, y_ctr, w, h) -> (x1, y1, x2, y2)# 左上角为中心点，形成9个anchoranchors[:, 0::2] -= np.tile(anchors[:, 2] * 0.5, (2, 1)).Tanchors[:, 1::2] -= np.tile(anchors[:, 3] * 0.5, (2, 1)).Treturn anchors

演示步骤与效果如下所示：

pyramid_levels = [3, 4, 5, 6, 7]
strides = [2 ** x for x in pyramid_levels]
sizes = [2 ** (x + 2) for x in pyramid_levels]
ratios = np.array([0.5, 1, 2])
scales = np.array([2 ** 0, 2 ** (1.0 / 3.0), 2 ** (2.0 / 3.0)])

num_anchors = len(ratios) * len(scales)
anchors = np.zeros((num_anchors, 4))

anchors[:, 2:] = 16 * np.tile(scales, (2, len(ratios))).T
print("anchor的w、h的基础大小（1：1）: ")
print(anchors[:, 2:])

areas = anchors[:, 2] * anchors[:, 3]
print("基础面积：" )
print(areas)

anchors[:, 2] = np.sqrt(areas / np.repeat(ratios, len(scales)))
anchors[:, 3] = anchors[:, 2] * np.repeat(ratios, len(scales))
print("宽度：")
print(anchors[:, 2])
print("高度：")
print(anchors[:, 3])

anchors[:, 0::2] -= np.tile(anchors[:, 2] * 0.5, (2, 1)).T
anchors[:, 1::2] -= np.tile(anchors[:, 3] * 0.5, (2, 1)).T
print("一个位置生成的anchor如下")
print("个数为：",anchors.shape[0] )
print(anchors)

2.为每个位置生成anchor

基本思想还是：anchor的生成都是以原图为基准的

想要实现上述思想，最重要的就是得到特征图与原图的缩放比例（步长），比如stride=8，那么如果原图大小为（image_w,image_h)那么特征图相对于原图尺寸就缩小为(image_w/8 , image_h/8)(计算结果是上采样的）

那么每个anchor的位置是由特征图决定的

x1∈（ 0，1，2，3......image_w/8) y1∈（ 0，1，2，3......image_h/8)

生成anchor的位置就是 c_x1 = x1+0.5 ，c_y1 = y1+0.5

因为anchor的生成是以原图为基准的，所以要将anchor在特征图的位置放大到原图，即在原图上生成anchor的位置是 c_x = c_x1 * stride , c_y = c_y1 * stride

def shift(shape, stride, anchors):shift_x = (np.arange(0, shape[1]) + 0.5) * strideshift_y = (np.arange(0, shape[0]) + 0.5) * strideshift_x, shift_y = np.meshgrid(shift_x, shift_y)shifts = np.vstack((shift_x.ravel(), shift_y.ravel(),shift_x.ravel(), shift_y.ravel())).transpose()# add A anchors (1, A, 4) to# cell K shifts (K, 1, 4) to get# shift anchors (K, A, 4)# reshape to (K*A, 4) shifted anchorsA = anchors.shape[0]K = shifts.shape[0]all_anchors = (anchors.reshape((1, A, 4)) + shifts.reshape((1, K, 4)).transpose((1, 0, 2)))all_anchors = all_anchors.reshape((K * A, 4))return all_anchors

在代码层面上(anchors.reshape((1, A, 4)) + shifts.reshape((1, K, 4)).transpose((1, 0, 2)))

用到了向量相加的广播机制

向量a1维度是（k，1，4），含义是有K个位置，每个位置1份数据，每份数据4个参数（中心点）

向量a2维度是（1，A，4），含义是1个位置，每个位置A份数据，每份数据4个参数（anchor相对于中心点的位置坐标）

其中k是要在图像的k个位置上生成anchor，A是每个位置生成几个anchor

首先a2要在第0维复制k次（A,4）向量（为每个位置复制）

然后a1要在第1维复制A次（4）向下（为每个位置的每个anchor复制）

3.图片的anchor生成过程

最后输出的形状是 [1, 每层anchor数量之和 , 4]

class Anchors(nn.Module):def __init__(self, pyramid_levels=None, strides=None, sizes=None, ratios=None, scales=None):super(Anchors, self).__init__()#提取的特征if pyramid_levels is None:self.pyramid_levels = [3, 4, 5, 6, 7]#步长，在每层中，一个像素等于原始图像中几个像素if strides is None:self.strides = [2 ** x for x in self.pyramid_levels]   #这个参数设置我没看懂#每层框子的基本边长if sizes is None:self.sizes = [2 ** (x + 2) for x in self.pyramid_levels]  #这个参数设置我也没看懂#长宽比例if ratios is None:self.ratios = np.array([0.5, 1, 2])#边长缩放比例if scales is None:self.scales = np.array([2 ** 0, 2 ** (1.0 / 3.0), 2 ** (2.0 / 3.0)])def forward(self, image):#image是原图 shape为 batch，channel，w，h#这一步是获得宽和高image_shape = image.shape[2:]image_shape = np.array(image_shape)#‘//’是向下取整 整个式子相当于向上取整，因为不满1步的也要算1步#图像大小除以步长#在对应的每一层中，原图在该层对应的大小image_shapes = [(image_shape + 2 ** x - 1) // (2 ** x) for x in self.pyramid_levels]# 创建x1，y1 x2，y2 anchor的位置坐标all_anchors = np.zeros((0, 4)).astype(np.float32)for idx, p in enumerate(self.pyramid_levels):#传入该层anchor的基本边长，生成对应大小的anchoranchors = generate_anchors(base_size=self.sizes[idx], ratios=self.ratios, scales=self.scales)# 传入生成的anchor，和该层相对于原图的大小shifted_anchors = shift(image_shapes[idx], self.strides[idx], anchors)# 循环遍历完成之后，all_anchors的shape为 [每层anchor数量之和， 4]all_anchors = np.append(all_anchors, shifted_anchors, axis=0)# 最后输出的形状是 [1, 每层anchor数量之和 , 4]all_anchors = np.expand_dims(all_anchors, axis=0)if torch.cuda.is_available():return torch.from_numpy(all_anchors.astype(np.float32)).cuda()else:return torch.from_numpy(all_anchors.astype(np.float32))